The purpose of this study is to investigate glucose intolerance in cystic fibrosis. We have noted a prevalence of 85.7% of abnormal post-prandial blood glucose and insulin values in the Cystic Fibrosis population seen at Duke. In order to characterize the mechanism of this abnormality we are using an intravenous glucose tolerance test (in the manner of Bergman) to measure insulin resistance before and after dietary manipulation.